The present invention relates to a method and circuit arrangement for compensating nonlinearities of transmission members in a radio relay transmission system by a converter operating in the linear and distorting, or harmonic, ranges.
The transmission of amplitude modulated signals over radio relay paths requires high linearity in the microwave transmitting amplifier. This linearity cannot be realized in the amplifier itself.
The problem can be solved in two ways: firstly, according to the predistortion method; and secondly according to the older preliminary deaccentuation, or feed forward, method.
Usually, the latter type of linearization is not used for broadband deaccentuator circuits because of the high costs involved.
Circuits operating according to the feed forward principle are used either in the intermediate frequency plane or in the radio frequency plane, the latter having a smaller relative bandwidth but usually also high transmission losses of about 7 to 10 dB which must be compensated with an additional microwave amplifier, for example an FET amplifier.
German Offenlengungsschrift [Laid-Open Application] No. 2,923,046 and corresponding U.S. Application Ser. No. 157,172 filed by Weber on June 6th, 1980, now U.S. Pat. No. 4,346,479, issued Aug. 24, 1982, also discloses a converter arrangement with predistortion in the RF plane in which the harmonic generator converter receives the local oscillator signal via a level and phase regulator and the converter output signals are combined in a summing member. A drawback of this circuit arrangement, however, is that, depending on the phase position of the annoying nonlinearity, the phase of the oscillator signal must be shifted for the harmonic generator converter to such an extent that the useful output signal of the harmonic generator converter reaches the range of the opposite phase to the HF output signal of the linear converter, which may likewise create high losses.